To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, extreme ultraviolet (EUV) lithography of 13.5 nm wavelength, and double patterning version of the ArF lithography, on which active research efforts have been made.
With respect to high-energy radiation of very short wavelength such as electron beam (EB) or x-ray, hydrocarbons and similar light elements used in resist materials have little absorption. Then polyhydroxystyrene base resist materials are under consideration. Resist materials for EB lithography are practically used in the mask image writing application. Recently, the mask manufacturing technology becomes of greater interest. Reduction projection exposure systems or steppers have been used since the time when the exposure light was g-line. While their demagnification factor was ⅕, a factor of ¼ is now used as a result of chip size enlargement and projection lens diameter increase. It becomes of concern that a dimensional error of a mask has an impact on the dimensional variation of a pattern on wafer. It is pointed out that as the pattern feature is reduced, the value of a dimensional variation on the wafer becomes greater than the value of a dimensional error of the mask. This is evaluated by a mask error enhancement factor (MEEF) which is a dimensional variation on wafer divided by a dimensional error of mask. Patterns on the order of 45 nm often show an MEEF in excess of 4. In a situation including a demagnification factor of ¼ and a MEEF of 4, the mask manufacture needs an accuracy substantially equivalent to that for equi-magnification masks.
The exposure system for mask manufacturing made a transition from the laser beam exposure system to the EB exposure system to increase the accuracy of line width. Since a further size reduction becomes possible by increasing the accelerating voltage of the electron gun in the EB exposure system, the accelerating voltage increased from 10 keV to 30 keV and reached 50 keV in the current mainstream system, with a voltage of 100 keV being under investigation.
As the accelerating voltage increases, a lowering of sensitivity of resist film becomes of concern. As the accelerating voltage increases, the influence of forward scattering in a resist film becomes so reduced that the contrast of electron image writing energy is improved to ameliorate resolution and dimensional control whereas electrons can pass straightforward through the resist film so that the resist film becomes less sensitive. Since the mask exposure tool is designed for exposure by direct continuous writing, a lowering of sensitivity of resist film leads to an undesirably reduced throughput. Due to a need for higher sensitivity, chemically amplified resist compositions are contemplated.
Thinning of resist film is in progress to facilitate reduction of pattern feature in the EB lithography for mask manufacturing and to prevent the pattern from collapsing due to a higher aspect ratio during development. In the case of photolithography, a thinning of resist film greatly contributes to resolution improvement. This is because introduction of chemical mechanical polishing (CMP) or the like has driven forward device planarization. In the case of mask manufacture, substrates are flat, and the thickness of processable substrates (e.g., Cr, MoSi or SiO2) is predetermined by a percent light shield or phase shift control. The dry etch resistance of resist film must be improved before the film can be reduced in thickness.
It is generally believed that there is a correlation between the carbon density and the dry etch resistance of resist film. For EB writing which is not affected by absorption, resist materials based on novolac resins having better etch resistance have been developed. Indene copolymers described in Patent Document 1 and acenaphthylene copolymers described in Patent Document 2 are expected to have improved etch resistance due to a high carbon density and a robust main chain structure based on cycloolefin structure.
Also, with respect to the soft x-ray (EUV) lithography at wavelength 5-20 nm, the reduced absorption of carbon atoms was reported. Increasing the carbon density is effective not only for improving dry etch resistance, but also for increasing the transmittance in the soft x-ray wavelength region.
As the feature size is reduced, image blurs due to acid diffusion become a problem. To insure resolution for fine patterns with a size of 45 nm et seq., not only an improvement in dissolution contrast is requisite, but control of acid diffusion is also important, as known from previous reports. Since chemically amplified resist compositions are designed such that sensitivity and contrast are enhanced by acid diffusion, an attempt to minimize acid diffusion by reducing the temperature and/or time of post-exposure baking (PEB) fails, resulting in drastic reductions of sensitivity and contrast. Since the distance of acid diffusion is closely related to the type of acid labile group, it would be desirable to have an acid labile group which permits deprotection reaction to proceed at a very short distance of acid diffusion.
A tradeoff among sensitivity, edge roughness and resolution is reported. Increasing sensitivity leads to reductions of edge roughness and resolution. Controlling acid diffusion improves resolution at the sacrifice of edge roughness and sensitivity. Addition of an acid generator capable of generating a bulky acid is effective for suppressing acid diffusion, but leads to reductions of edge roughness and sensitivity as pointed out above. It is then proposed to copolymerize a polymer with an acid generator in the form of an onium salt having polymerizable olefin. Patent Documents 3 to 5 disclose sulfonium salts having polymerizable olefin capable of generating a sulfonic acid and similar iodonium salts. A photoresist using a base polymer having a polymerizable acid generator copolymerized therein exhibits reduced edge roughness due to controlled acid diffusion and uniform dispersion of acid generator within the polymer, succeeding in improving both resolution and edge roughness at the same time.
Patent Documents 6 and 7 disclose resist compositions comprising polymer comprising recurring units having an acid generator and recurring units in which hydroxyl of hydroxystyrene or carboxyl of methacrylate is substituted with an acid labile group. Suitable acid labile groups for hydroxystyrene include acetal, linear tertiary ether, ethylcyclohexyl, and tert-butoxycarbonyl (t-BOC). Of these, the acetal compounds are so sensitive to acid that they undergo deprotection reaction upon EB or EUV exposure in vacuum, resulting in outgassing. The linear ether and t-BOC groups undergo slow deprotection reaction, but still suffer from the outgassing problem because components resulting from deprotection are volatile. It would be desirable to have an acid labile group on hydroxystyrene featuring minimal outgassing.